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It is sometimes useful to be able to separate those parts of a signal that are periodic with respect to some function of time from the parts that are non-periodic with respect to time. This ability is commonly used in the analysis of rotating machinery such as spindles and computer disk-drive motors. In the machine tool industry and the disk-drive industry, such non-periodic motion is usually referred to as asynchronous error motion or non-repeatable run-out. In fact, these measurements are so well known that both IDEMA (the trade association for the data storage industry) and ANSI (American National Standards Institute) have specifications (T17-91 and B89.3.4.M, respectively) describing how to perform the measurements.
As an example of why such measurements are important, consider the radial run-out of a ball-bearing spindle. The run-out signal will contain certain components that are periodic with respect to the spindle""s rotation rate. These signals would include a component at the rotation rate caused by imperfect centering, as well as frequency components at integral numbers of the rotation rate caused by imperfect circularity. Non-periodic components will also be present. The rate at which the balls in the ball bearing precess around the shaft commonly causes an asynchronous signal as do non-circularity of the balls and vibrations of the spindle.
In the prior art, these measurements, as typified by IDEMA spec T17-91, are made by synchronously sampling the radial or axial position of the rotating part at a rate many times its rotational rate, over some number of revolutions. Because the sampling is synchronous with the rotational rate, the samples repetitively occur at fixed spots on the sampled part. So, if the part were sampled N times per revolution, there would be N spots at equal angular spacing around the part that are sampled once per revolution. The peak-to-peak variation in the reading obtained at each spot over the sampled revolutions represents the asynchronous run-out at that spot. The average of all readings at each spot represents the synchronous position at that spot. The peak-to-peak value of all synchronous positions is often used to calculate the synchronous error motion, or synchronous TIR. The largest of the asynchronous error motions as measured at each spot, is often considered to be the asynchronous error motion of the unit under test.
This method of calculating synchronous and asynchronous error motion by using synchronous sampling has several disadvantages. First, different angular rotation rates, or different numbers of samples per revolution require different sample rates. This requirement increases the cost and complexity of the equipment used to make the samples. In addition, it also increases the complexity of digitally filtering the sampled data, which is often desirable.
Another problem is the generation of the trigger signals used to make the synchronous samples. It is most advantageous to have the signals generated by the rotating part itself such as could be accomplished by using a rotary encoder connected to the rotating part. However, it is often impossible to use this technique, and instead, the trigger signals are commonly generated by an electronic circuit. Such circuits are fed a once-per-revolution pulse, and from this, generate N pulses per revolution by means of some frequency multiplication technique. Such techniques can have jitter and cannot precisely follow variations in the rotating part""s angular speed. In addition, they add considerable cost and complexity to the system.
A more subtle problem is the under calculation of asynchronous signals at certain frequencies relative to the rotational frequency. Consider an asynchronous signal that occurs at ⅔rds the rotational frequency. Although such a signal is non-periodic and is considered to be part of asynchronous error motion, we note that 3 times ⅔rds equals 2, so the signal at this frequency will cause a pattern that repeats every three revolutions. Because of this repetitive pattern, the asynchronous error motion of such a signal will be under calculated if the synchronous sampling technique is used.
This patent teaches a new method that uses asynchronous sampling of the device under test. The technique eliminates all the disadvantages of the synchronous-sampling method discussed above. In the preferred embodiment of this method, a piece of rotating equipmentxe2x80x94the Device Under Test (DUT)xe2x80x94is instrumented with a machine that measures some physical characteristic of the DUT as a function of time. The data are sampled at a rate significantly faster than the rotational period of the DUT and logged to a computer for a number of revolutions. At the same time, another instrument very accurately measures the time each revolution of the DUT takes. This measurement is typically made by using a once-per-revolution pulse commonly available from rotating machinery. Some physical characteristics of the DUT depend solely upon the rotational position of the device, and will be periodic with the rotation. Other physical characteristics of the DUT are not periodic with the rotation. The goal of the signal processing module is to separate the periodic and non-periodic portions of the logged data. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.